Implantable medical device for improved storage of intracardiac electrograms

ABSTRACT

An implantable medical device provides for improved storage of recorded IEGMs. A sensing stage is connected to an electrode for picking up electric potentials from inside a heart, the time course of said electric potentials representing a heart signal, a control unit connected to said sensing stage is adapted to process a sequence of data points that each represent an amplitude or magnitude A of a time-varying signal at equidistant points of time t, wherein end points of data segments are determined by processing of the sequence of data points. The control unit is adapted to identify end points of data segments by processing of the sequence of data points.

FIELD OF INVENTION

The invention refers to an implantable medical device that storestime-variable signals that are sampled over time. The inventionparticularly refers to implantable pacemakers and implantablecardioverter/defibrillators featuring automatic capture threshold searchand to storing EGM signals in such implant.

BACKGROUND OF THE INVENTION

Implantable medical devices and in particular heart stimulators can beused for treating a variety of heart disorders like bradycardia,tachycardia or fibrillation by way of electric stimulation pulsesdelivered to the heart tissue, the myocardium. The ability of suchdevices to pick-up electrical potential in a heart is often times usedto acquire an intracardiac electrogram (IEGM) that is a heart signalrepresenting the time course of the electric potential picked up by theimplant.

Depending on the disorder to be treated, such heart stimulator generateselectrical stimulation pulses that are delivered to the heart tissue(myocardium) of a respective heart chamber according to an adequatetiming regime. Delivery of stimulation pulses to the myocardium isusually achieved by means of an electrode lead that is electricallyconnected to a stimulation pulse generator inside a heart stimulator'shousing and that carries a stimulation electrode in the region of itsdistal end. A stimulation pulse also is called a pace. Similarly, pacinga heart chamber means stimulating a heart chamber by delivery of astimulation pulse.

In order to be able to sense a contraction a heart chamber thatnaturally occurs without artificial stimulation and that is calledintrinsic, the heart stimulator usually comprises at least one sensingstage that is connected to a sensing electrode on said electrode placedin the heart chamber. An intrinsic excitation of a heart chamber resultsin characteristic electrical potentials that are picked up via thesensing electrode and that can be evaluated by the sensing stage inorder to determine whether an intrinsic excitation—called: intrinsicevent—has occurred.

Usually, a heart stimulator features separate stimulation generators foreach heart chamber to be stimulated. Therefore, in a dual chamberpacemaker, usually an atrial and a ventricular stimulation pulsegenerator for generating atrial and ventricular stimulation pulses areprovided. Delivery of an atrial or a ventricular stimulation pulsecausing an artificial excitation of the atrium or the ventricle,respectively, is called an atrial stimulation event A_(P) (atrial pacedevent) or a ventricular stimulation event V_(P) (ventricular pacedevent), respectively. The strength of stimulation pulses delivered bythe respective stimulation pulse generator is adjustable in order to beable to adjust the stimulation pulse strength to be just sufficient tocause capture (above capture threshold) and thus using as little energyas possible to be effective. Stimulation pulse strength depends on both,duration and amplitude of the stimulation pulse.

Common heart stimulators feature separate sensing stages for each heartchamber to be of interest. In a dual chamber pacemaker usually twoseparate sensing stages, an atrial sensing stage and a ventricularsensing stage, are provided that are capable to detect intrinsic atrialevents A_(S) (atrial sensed event) or intrinsic ventricular events V_(S)(ventricular sensed event), respectively.

As known in the art, separate sensing and pacing stages are provided forthree-chamber (right atrium RA, right ventricle RV, left ventricle LV)or four-chamber (right atrium RA, left atrium LA, right ventricle RV,left ventricle LV) pacemakers or ICDs.

By means of a sensing stage for a heart chamber to be stimulated, thepacemaker is able to only trigger stimulation pulses when needed that iswhen no intrinsic excitation of the heart chamber occurs in time. Suchmode of pacing a heart chamber is called demand mode. In the demand modethe pacemaker schedules an atrial or a ventricular escape interval thatcauses triggering of an atrial or ventricular stimulation pulse when theescape interval times out. Otherwise, if an intrinsic atrial orventricular event is detected prior to time out of the respective atrialor ventricular escape interval, triggering of the atrial or ventricularstimulation pulse is inhibited. Such intrinsic (natural, non-stimulated)excitation are manifested by the occurrence of recognizable electricalsignals that accompany the depolarization or excitation of a cardiacmuscle tissue (myocardium). The depolarization of the myocardium isusually immediately followed by a cardiac contraction. For the purposeof the present application, depolarization and contraction may beconsidered as simultaneous events and the terms “depolarization” and“contraction” are used herein as synonyms. The recognizable electricalsignals that accompany the depolarization or excitation of a heartchamber are picked up (sensed) by the atrial or the ventricular sensingchannel, respectively. Thus, by means of the sensing stages,intracardiac electrogram signals are acquired, that can be evaluated bythe implantable medical device. Simple evaluation only checks whetherthe IEGM exceeds a given threshold in order to detect a sense event.More complex evaluation includes analysis of the IEGM's morphology.

In order to allow for such morphology analysis, it is desirable torecord the time course of an IEGM signal by means of sampling thesignal. Sampling is carried out by measuring the signals amplitude atpredetermined points of time with a constant sampling interval.

In a heart cycle, an excitation of the myocardium leads todepolarization of the myocardium that causes a contraction of the heartchamber. If the myocardium is fully depolarized it is unsusceptible forfurther excitation and thus refractory. Thereafter, the myocardiumrepolarizes and thus relaxes and the heart chamber is expanding again.In a typical electrogram (EGM) depolarization of the ventriclecorresponds to a signal known as “R-wave”. The repolarization of theventricular myocardium coincides with a signal known as “T-wave”. Atrialdepolarization is manifested by a signal known as “P-wave”. Forevaluation of an IEGM it is desirable to be able to determine theseparticular signals.

With respect to capture control, it is further desirable to have arepresentation of an IEGM that allows for discrimination between apolarization artifact following an ineffective stimulation pulse and anevoked response following an effective stimulation pulse.

Several modes of operation are available in a state of the art multimode pacemaker. The pacing modes of a pacemaker, both single and dual ormore chamber pacemakers, are classified by type according to a threeletter code. In such code, the first letter identifies the chamber ofthe heart that is paced (i.e., that chamber where a stimulation pulse isdelivered), with a “V” indicating the ventricle, an “A” indicating theatrium, and a “D” indicating both the atrium and ventricle. The secondletter of the code identifies the chamber wherein cardiac activity issensed, using the same letters, and wherein an “O” indicates no sensingoccurs. The third letter of the code identifies the action or responsethat is taken by the pacemaker. In general, three types of action orresponses are recognized: (1) an Inhibiting (“I”) response wherein astimulation pulse is delivered to the designated chamber at theconclusion of the appropriate escape interval unless cardiac activity issensed during the escape interval, in which case the stimulation pulseis inhibited; (2) a Trigger (“T”) response wherein a stimulation pulseto a prescribed chamber of the heart a prescribed period of time after asensed event; or (3) a Dual (“D”) response wherein both the Inhibitingmode and Trigger mode may be evoked, e.g., with the “inhibiting”occurring in one chamber of the heart and the “triggering” in the other.

To such three letter code, a fourth letter “R” may be added to designatea rate-responsive pacemaker and/or whether the rate-responsive featuresof such a rate-responsive pacemaker are enabled (“O” typically beingused to designate that rate-responsive operation has been disabled). Arate-responsive pacemaker is one wherein a specified parameter orcombination of parameters, such as physical activity, the amount ofoxygen in the blood, the temperature of the blood, etc., is sensed withan appropriate sensor and is used as a physiological indicator of whatthe pacing rate should be. When enabled, such rate-responsive pacemakerthus provides stimulation pulses that best meet the physiologicaldemands of the patient.

A dual chamber pacemaker featuring an atrial and a ventricular sensingstage and an atrial and a ventricular stimulation pulse generator can beoperated in a number of stimulation modes like VVI, wherein atrial senseevents are ignored and no atrial stimulation pulses are generated, butonly ventricular stimulation pulses are delivered in a demand mode, AAI,wherein ventricular sense events are ignored and no ventricularstimulation pulses are generated, but only atrial stimulation pulses aredelivered in a demand mode, or DDD, wherein both, atrial and ventricularstimulation pulses are delivered in a demand mode. In such DDD mode ofpacing, ventricular stimulation pulses can be generated in synchronywith sensed intrinsic atrial events and thus in synchrony with anintrinsic atrial rate, wherein a ventricular stimulation pulse isscheduled to follow an intrinsic atrial contraction after an appropriateatrioventricular delay (AV-delay; AVD), thereby maintaining thehemodynamic benefit of atrioventricular synchrony.

From the foregoing it becomes apparent that there is a need to providethe physician with a graphical representation of an intracardiacelectrogram in order to facilitate heart diagnosis and optimize the modeof pacemaker operation.

The IEGM acquired by the implantable medical device can either be storedin the implant itself or be telemetrically transmitted to a centralservice center remote from the individual implant. For both cases it ispreferred to have as little data as possible to be stored ortransmitted. Therefore there is a general need for an effective datarepresentation of a time course of a signal such as an IEGM.

A method for an effective representation of an IEGM is known from U.S.Pat. No. 5,836,889. U.S. Pat. No. 5,836,889 discloses a method andapparatus that identifies turning points in an intracardiac EGM that issampled at equidistant time points by comparing the slope between anactual sample (n) value and the second last sample (n−1) value with theslope between the actual sample (n) value and the last identifiedturning point. If the difference between the two slopes thus determinedexceeds a predetermined threshold, the second last sample value ismarked as a further turning point of the EGM. Once all turning pointsare thus identified, only the turning points of the EGM signal arestored as a compressed data representation of the EGM signal whereasthose sample values not being identified as turning points can bediscarded. The slope is determined by determining the differencequotient between two samples. The difference quotient for an actualsample and the second last sample approximately corresponds to the firstderivative with respect to time of the EGM signal, because the actualsample and the second last sample are immediate neighbours. Thedisclosure of U.S. Pat. No. 5,836,889 is included herein by reference.

SUMMARY OF THE INVENTION

It is an object of the invention to provide

an implantable medical device that provides for improved storage ofrecorded IEGMs.

According to the present invention the object of the invention isachieved by an implantable medical device featuring:

a sensing stage connected or being connectable to an electrode forpicking up electric potentials inside at least said ventricle of aheart, the time course of said electric potentials representing a heartsignal,

and

a control unit that is connected to said sensing stage.

The control unit is adapted to process a sequence of data points thateach represent an amplitude or magnitude A of a time-varying signal atequidistant points of time t, wherein end points of datasegments aredetermined by processing of the sequence of data points.

The control unit is adapted to identify end points of data segments by

a) determination of a first difference quotientD₁=(A_(n)−A_(n−1))/(t_(n)−t_(n−1)) with respect to an actual data pointA_(n) at t_(n) and an immediately preceding, second last data pointA_(n−1) at t_(n−1),

b) determination of second difference quotientD₂=(A_(n)−A_(e))/(t_(n)−t_(e)) with respect to an actual data pointA_(n) at t_(n) and a last end point A_(e) at t_(e), said last end pointA_(e) being a previously determined end point or a first data point ofsaid sequence of data points, wherein t_(e) represents the point of timebelonging to said end point,c) selecting the second last data point A_(n−1) as a new end point, ifthe magnitude (the absolute value) of the difference between the twodifference quotients D_(D)=D₁−D₂ exceeds a predetermined first thresholdvalue T, D_(D)>T, andd) determining a data segment length L=t_(n)−t_(e) between the point oftime t_(n) of an actual data point A_(n) and the point of time t_(e) ofa last end point A_(e),e) and selecting A_(n−1) as a new end point, if said data segment lengthL exceeds a predetermined maximum length L_(max).

Alternatively, the second difference quotient can be determined based ona second last data point A_(n−1) and t_(n−1) and a last end point A_(e)at t_(e):D ₂=(A _(n) −A _(e))/(t _(n) −t _(e))

Thus, the control unit selects those data points as end points of a datasegment, that either meet the selection criteria a), b) and c) or thatmeet the selection criteria d) and e).

The control unit is further adapted to store every selected end point inassociation with a segment length between each stored endpoint and animmediately preceding end point as a compressed data representation ofsaid time varying signal.

The difference quotient represents the slope or the first derivative ofa straight line crossing the two data points (A_(n), t_(n); A_(n−1),t_(n−1) or A_(n), t_(n); A_(e), t_(e), respectively) for which thedifference quotient is determined.

The segment length can be expressed by the number of sampling intervalsbetween the two endpoints of a segment.

The rules for selecting endpoints can alternatively be expressed asfollows:

a) a partial quantity of the said obtained signal samples is selectedfor storage and/or transmission using a set of selection criteria;

b) the said set of selection criteria uses the first derivatives of thesignal (or, to be more precise: of a straight line crossing to datapoints of a time series representing the signal), and for each sampleunder consideration, two such derivatives are calculated—the currentderivative and the segment derivative;c) wherein the said current derivative is that of the straight-lineconnection between the (n)th and the (n−1)th signal sample—(n)th samplebeing the sample under consideration,d) and the said segment derivative is that of the straight-lineconnection between the (n)th signal sample and the last-stored and/orlast-transmitted signal sample;e) wherein the said set of selection criteria consists of at least thefollowing three rules

-   -   i. the storage condition is met if the magnitude of the        difference of the current and the segment derivatives is above a        predetermined limit (T),    -   ii. the storage condition is met if the said current derivative        is zero and the magnitude of the said segment derivative is        above a predetermined limit (T1),    -   iii. the storage condition is met if the segment        duration—between the last-stored and/or last-transmitted signal        sample and the (n−1)th sample—reaches the value identified as        the maximum number that can be stored or transmitted in the        allocated ‘length’ portion of the storable and/or transmittable        data word;        f) having found any one of the said rules being met, the (n−1)th        signal sample is stored and/or transmitted and is designated as        the new last-stored and/or last-transmitted signal sample,        g) and the said segment duration is also stored and/or        transmitted in the allocated ‘length’ portion of the data word.

It is to be noted that threshold values T and T1 may be identical, sinceii. is a special case of i.

Preferably, the control unit is further adapted to determine whethersaid first and said second difference quotient are of opposite sign andselect the second last data point A_(n−1) as a new end point, if thedifference between the two difference quotients D_(D)=D₁−D₂ exceeds apredetermined second threshold value T2.

In other words: a further storage condition met if the said current andsegment derivatives are of opposite polarities and the magnitude of thedifference of the current and the segment derivatives is above anotherpredetermined lower limit T2.

The control unit can be further adapted to determine whether a datasegment length L=t_(n)−t_(e) between the point of time t_(n) of anactual data point A_(n) and the point of time t_(e) of a last end pointA_(e) exceeds a predetermined threshold length L1 and whether thedifference between the two difference quotients D_(D)=D₁−D₂ exceeds apredetermined third threshold value T3, and to select the second lastdata point A_(n−1) as a new end point, if said two conditions are met.

Regarding the need to set adequate threshold values, the control unit ispreferably adapted to determine the first threshold value T, and, ifapplicable, the second and the third threshold value as a predeterminedfraction of a peak amplitude value of said time-varying signal and toredetermine said threshold values if a moving average of said peakamplitude value of said time-varying signal changes.

Depending on whether or not T and T1 are identical, according to apreferred embodiment of the invention, three or four threshold valuesare to be determined as pointed out above.

In a two channel device providing two sensing stages, e.g. a ventricularsensing stage and an atrial sensing stage, that generate two datasequences, it is preferred to generate a combined compressed datasequence comprising all selected endpoints of both of the datasequences. Therefore, the control unit preferably is adapted to selectendpoints of both data sequences and to store every selected endpoint ofany of the data sequences together with a corresponding data point ofthe other data sequence and the segment length between to consecutiveendpoints irrespective the data sequence the endpoints belong to. It canbe said that thus every data point of one data sequence that correspondsto an endpoint of the other data sequence becomes an endpoint itself—orat least is treated like an endpoint even if it does not fulfill theselection criteria itself.

In order to achieve a further data compression it is preferred that thecontrol unit is adapted to transform each amplitude value (in otherwords: each selected data point) into a transformed amplitude valueprior to storing a selected end point, wherein the transformation isnonlinear such that endpoint values with lower magnitudes are encodedwith greater precision, while endpoint values with higher magnitudes areencoded with lesser precision.

With respect to possible downsampling of a sequence of data points it ispreferred that the control unit is adapted to downsample an originaldata sequence in order to generate a data sequence comprising less datapoints than the original data sequence by dropping one or more points ofthe original data sequence wherein a point having the largest magnitudeof all these mentioned points is retained and the others are dropped,such that all data points representing peak values of the original datasequence are persevered in the downsampled data sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 shows a dual chamber pacemaker connected to leads placed in aheart.

FIG. 2 is a block diagram of a heart stimulator according to theinvention.

FIG. 3 is a schematic, graphic representation of an EKG signal, alongwith its representation according to the method of the invention.

FIG. 4 is graphical representation of a first criterion to select asegment end point according to the invention.

FIG. 5 is a representation of two alternative data formats for storingselected end points according to the invention.

FIG. 6 is a flow chart illustrating selection of ‘Endpoints’ for asingle data sequence.

FIG. 7 is a transformation table for nonlinear transformation ofamplitude values according to a preferred embodiment of the invention.

FIG. 8 is a graphical representation of the nonlinear transformationaccording to FIG. 7.

FIG. 9 is a flow chart illustrating selection of ‘Endpoints’ in a dualchannel device such as the dual chamber pacemaker of FIG. 1.

FIG. 10 is an example of an original atrial IEGM together with arepresentation of the compressed atrial IEGM. The compressed IEGM ismarked T+.

FIG. 11 is an example of an original ventricular IEGM together with arepresentation of the compressed ventricular IEGM.

FIG. 12 is another example of an original atrial IEGM together with arepresentation of the compressed atrial IEGM.

FIG. 13 is a further example of an original atrial IEGM together with arepresentation of the compressed atrial IEGM.

FIG. 14 is a further example of an original ventricular IEGM togetherwith a representation of the compressed ventricular IEGM.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

In FIG. 1 a dual chamber pacemaker 10 as heart stimulator connected topacing/sensing leads placed in a heart 12 is illustrated. The pacemaker10 is electrically coupled to heart 12 by way of leads 14 and 16. Lead14 has a pair of right atrial electrodes 18 and 20 that are in contactwith the right atria 26 of the heart 12. Lead 16 has a pair ofelectrodes 22 and 24 that are in contact with the right ventricle 28 ofheart 12. Electrodes 18 and 22 are tip-electrodes at the very distal endof leads 14 and 16, respectively. Electrode 18 is a right atrial tipelectrode RA-Tip and electrode 22 is a right ventricular tip electrode22. Electrodes 20 and 24 are ring electrodes in close proximity butelectrically isolated from the respective tip electrodes 18 and 22.Electrode 20 forms a right atrial ring electrode RA-Ring and electrode24 forms a right ventricular ring electrode RV-Ring.

Referring to FIG. 2 a simplified block diagram of a dual chamberpacemaker 10 is illustrated. During operation of the pacemaker leads 14and 16 are connected to respective output/input terminals of pacemaker10 as indicated in FIG. 1 and carry stimulating pulses to the tipelectrodes 18 and 22 from an atrial stimulation pulse generator A-STIM32 and a ventricular pulse generator V-STIM 34, respectively. Further,electrical signals from the atrium are carried from the electrode pair18 and 20, through the lead 14, to the input terminal of an atrialchannel sensing stage A-SENS 36; and electrical signals from theventricles are carried from the electrode pair 22 and 24, through thelead 16, to the input terminal of a ventricular sensing stage V-SENS 38.

Controlling the dual chamber pacer 10 is a control unit CTRL 40 that isconnected to sensing stages A-SENS 36 and V-SENS 38 and to stimulationpulse generators A-STIM 32 and V-STIM 34. Control unit CTRL 40 receivesthe output signals from the atrial sensing stage A-SENS 32 and from theventricular sensing stage V-SENS 34. The output signals of sensingstages A-SENS 32 and V-SENS 34 are generated each time that a P-waverepresenting an intrinsic atrial event or an R-wave representing anintrinsic ventricular event, respectively, is sensed within the heart12. An As-signal is generated, when the atrial sensing stage A-SENS 32detects a P-wave and a Vs-signal is generated, when the ventricularsensing stage V-SENS 34 detects an R-wave.

Atrial and ventricular stimulation pulse generators A-STIM 36 and V-STIM38, respectively, are adapted to generate electrical stimulation pulseshaving an adjustable strength that depends on a control signal receivedfrom the control unit CTRL 40.

Control unit CTRL 40 also generates trigger signals that are sent to theatrial stimulation pulse generator A-STIM 36 and the ventricularstimulation pulse generator V-STIM 38, respectively. These triggersignals are generated each time that a stimulation pulse is to begenerated by the respective pulse generator A-STIM 36 or V-STIM 38. Theatrial trigger signal is referred to simply as the “A-pulse”, and theventricular trigger signal is referred to as the “V-pulse”. During thetime that either an atrial stimulation pulse or ventricular stimulationpulse is being delivered to the heart, the corresponding sensing stage,A-SENS 32 and/or V-SENS 34, is typically disabled by way of a blankingsignal presented to these amplifiers from the control unit CTRL 40,respectively. This blanking action prevents the sensing stages A-SENS 32and V-SENS 34 from becoming saturated from the relatively largestimulation pulses that are present at their input terminals during thistime. This blanking action also helps prevent residual electricalsignals present in the muscle tissue as a result of the pacerstimulation from being interpreted as P-waves or R-waves.

Furthermore, atrial sense events As recorded shortly after delivery of aventricular stimulation pulses during a preset time interval called postventricular atrial refractory period (PVARP) are generally recorded asatrial refractory sense event A_(rs) but ignored.

Control unit CTRL 40 comprises circuitry for timing ventricular and/oratrial stimulation pulses according to an adequate stimulation rate thatcan be adapted to a patient's hemodynamic need as pointed out below.

Still referring to FIG. 2, the pacer 10 may also include a memorycircuit MEM 42 that is coupled to the control unit CTRL 40 over asuitable data/address bus ADR 44. This memory circuit MEM 42 allowscertain control parameters, used by the control unit CTRL 40 incontrolling the operation of the pacemaker 10, to be programmably storedand modified, as required, in order to customize the pacemaker'soperation to suit the needs of a particular patient. Such data includesthe basic timing intervals used during operation of the pacemaker.Further, data sensed during the operation of the pacer may be stored inthe memory MEM 42 for later retrieval and analysis.

A telemetry circuit TEL 46 is further included in the pacemaker 10. Thistelemetry circuit TEL 46 is connected to the control unit CTRL 40 by wayof a suitable command/data bus. Telemetry circuit TEL 46 allows forwireless data exchange between the pacemaker 10 and some remoteprogramming or analyzing device which can be part of a centralizedservice center serving multiple pacemakers.

The pacemaker 10 in FIG. 1 is referred to as a dual chamber pacemakerbecause it interfaces with both the right atrium 26 and the rightventricle 28 of the heart 12. Those portions of the pacemaker 10 thatinterface with the right atrium, e.g., the lead 14, the P-wave sensingstage A-SENS 32, the atrial stimulation pulse generator A-STIM 36 andcorresponding portions of the control unit CTRL 40, are commonlyreferred to as the atrial channel. Similarly, those portions of thepacemaker 10 that interface with the right ventricle 28, e.g., the lead16, the R-wave sensing stage V-SENS 34, the ventricular stimulationpulse generator V-STIM 38, and corresponding portions of the controlunit CTRL 40, are commonly referred to as the ventricular channel.

In order to allow rate adaptive pacing in a DDDR or a DDIR mode, thepacemaker 10 further includes a physiological sensor ACT 48 that isconnected to the control unit CTRL 40 of the pacemaker 10. While thissensor ACT 48 is illustrated in FIG. 2 as being included within thepacemaker 10, it is to be understood that the sensor may also beexternal to the pacemaker 10, yet still be implanted within or carriedby the patient. A common type of sensor is an activity sensor, such as apiezoelectric crystal, mounted to the case of the pacemaker. Other typesof physiologic sensors are also known, such as sensors that sense theoxygen content of blood, respiration rate, pH of blood, body motion, andthe like. The type of sensor used is not critical to the presentinvention. Any sensor capable of sensing some physiological parameterrelatable to the rate at which the heart should be beating can be used.Such sensors are commonly used with “rate-responsive” pacemakers inorder to adjust the rate of the pacemaker in a manner that tracks thephysiological needs of the patient.

Now the operation of pacemaker 10 shall be illustrated.

Control unit CTRL 40 is adapted to perform a compression algorithm thatincludes determination and selection of end points of segments in aseries of data (data sequence) that represents the course of time of asignal at equidistant points of time, wherein each data point of thedata sequence represents the signal amplitude at the respective point oftime.

The compression algorithm is based on the assertion that if a series ofdata points can be represented by a single straight line segment forwhich the location of the line at the points of the intermediate samplescannot deviate from the actual sample values by more than a definedmaximum amount, then the intermediate samples can be ignored, and onlythe values of the endpoints of such segment, and the time between theend points (that is the segment length as represented by the number ofsampling intervals forming the segment), need be stored to represent thesignal with acceptable accuracy. This is illustrated in FIG. 3.

The identification of the ‘Endpoints’ is primarily based on identifyingchanging slope, and secondarily on a maximum segment length.

FIG. 4 illustrates how a new ‘Endpoint’ is identified using one of thecriteria described later on. To test for this criterion, the controlunit CTRL compares the slope defined by the current (n) and previous(n−1) data samples, to the slope defined by the current data sample (orthe previous data sample) and the last identified ‘Endpoint’ (e). If themagnitude of the difference between these two slopes is equal to orgreater than a defined threshold, then the previous data sample isidentified as the new ‘Endpoint’. Calculation of the slopes is performedby determination of the corresponding difference quotient.

In order to make best use of the physical memory, and to maintaincompression efficiency, each ‘Endpoint’ value that corresponds to theheart signal's magnitude at the particular point of time and itsassociated segment length are combined in a single data word. Each dataword has an endpoint value portion—or, in a dual chamber device, twoendpoint value portions—and a segment length portion. This is describedfor both single channel and dual channel implementations in FIG. 5.

By writing all zeroes in the ‘Length’ portion in the data word, the‘Endpoint Value’ portion of the data word can be used to include usefulinformation such as event identifiers or ‘markers’ in the compresseddata stream. If this data consolidation requires a reduction of thenumber of endpoint value codes available, a non-linear quantizationfunction is used to represent low valued data points with higherprecision, and higher valued data points with lower precision. Thisquantization function is not specifically described in the FIG. 5, butis discussed in more detail later on with respect to FIGS. 7 and 8.

The compression algorithm is suitable for implementation via embeddedsoftware, dedicated hardware, or a combination of the two. In any ofthese cases, the sequence of operations is the same, and is as describedin FIG. 6. The differences lie in the trade-offs between hardware andsoftware resources, and power consumption. In FIG. 6, the letter ‘X’ hasbeen used to identify the source of the signal, e.g. it can be replacedwith an ‘A’ for atrial signal or with a ‘V’ for ventricular signal asprovided by the atial sensing stage or the ventricular sensing stage,respectively.

The non-linear coding, referred to in the text associated with FIG. 5,is a means of dealing with EGM sample bit widths, which might be greaterthan the available bit-field width in the stored code word. The idea isthat endpoint values with lower magnitudes are encoded with greaterprecision, while endpoint values with higher magnitudes are encoded withlesser precision. The table in FIG. 7 shows an example of this type offunction:

This value-to-code mapping function is further illustrated in FIG. 8.

When storing two or more signals simultaneously, as, for example, couldbe useful in a dual chamber pacemaker, the compression method can beused in either of two ways:

-   1. Provide separate, independent compression systems to operate on    the individual channels. This would result in the greatest overall    compression. However, with this approach, the stored data is no    longer synchronized, resulting in the requirement of independent    memory buffers for the separate channels, as well as more hardware    and software tasks for managing the data.-   2. Use a single compression system with multiple data pipelines, and    enforce synchronization by artificially causing a new ‘Endpoint’ in    all channels whenever an endpoint criterion is met in one of the    channels. This conserves hardware and software resources, at the    expense of some compression.

When using the first of these approaches, the flowchart of FIG. 6applies directly to each channel independently. However, when using thesecond approach, that flowchart is enhanced, and is shown in FIG. 9 forthe case of two channels.

The value used for ‘Threshold’ (TX, TA and TV in the flowcharts in FIGS.6 and 9) determines the compression efficiency as well as the quality ofthe reproduced signal. A smaller value means better quality but lowercompression and a larger value means worse quality but highercompression. In the described embodiment, the ‘Threshold’ value for theindividual channel is calculated as a percentage of the peak value ofthe signal. Furthermore, this value is updated as the signal amplitudevaries from one detected heart complex to the next.

When processing the signal data with a goal of storing the compressedsignal at a lower sampling rate than the one used for the input signal,the larger of the sample values is retained as against going for a puredecimation where every n:th (n=2 when downsampling to half the samplingrate) sample is retained and all intermediate values simply dropped.This approach helps in retaining the peaks of the signal—of course,together with the other criteria as illustrated in the flowcharts.

The FIGS. 10 to 14 show some of the resulting compressed signals(labelled as T+) together with the original signal (labelled as ‘Org’).The FIGS. 10 to 12 show processing at half of the sampling rate whereasthe FIGS. 13 and 14 show processing at full sampling rate. Note that theoriginal signal is always displayed at full sampling rate.

1. An implantable medical device comprising: at least a sensing stageconnected or being connectable to an electrode for picking up electricpotentials from inside a heart, the time course of said electricpotentials representing a heart signal, said heart signal being sampledin constant sampling intervals and a control unit that is connected tosaid sensing stage and that is adapted to process a sequence of datapoints that each represent a magnitude A of a time-varying signal atequidistant points of time t, said processing of the sequence of datapoints includes: determining end points of data segments by a)determination of a first difference quotientD₁=(A_(n)−A_(n-1))/(t_(n)−t_(n-1)) with respect to an actual data pointA_(n) at t_(n) and an immediately preceding, second last data pointA_(n-1) at t_(n-1), b) determination of a second difference quotient D₂(A_(n)−A_(e))/(t_(n)−t_(e)) or D₂=(A_(n-1)−A_(e))/(t_(n-1)−t_(e)) withrespect to an actual data point A_(n) at t_(n) or a second last datapoint A_(n-1) and t_(n-1), respectively, and a last end point A_(e) att_(e), said last end point A_(e) being a previously determined end pointor a first data point of said sequence of data points, wherein t_(e)represents the point of time belonging to said end point A_(e), c)selecting the second last data point A_(n-1) as a new end point, if themagnitude of the difference between the two difference quotientsD_(D)=D₁−D₂ exceeds a predetermined first threshold value T, D_(D)>T,and determining a data segment length L=t_(n)−t_(e) between the point oftime t_(n) of an actual data point A_(n) and the point of time t_(e) ofa last end point A_(e), and selecting A_(n-1) as a new end point, ifsaid data segment length L exceeds a predetermined maximum lengthL_(max), and storing said selected end points in association with asegment length between each stored endpoint and an immediately precedingend point as a compressed data representation of said time varyingsignal.
 2. The implantable medical device of claim 1, wherein thecontrol unit is further adapted to: determine whether said first andsaid second difference quotient are of opposite sign and select thesecond last data point A_(n-1) as a new end point, if the magnitude ofthe difference between the two difference quotients D_(D)=D₁−D₂ exceedsa predetermined second threshold value T2.
 3. The implantable medicaldevice of claim 1 or 2, wherein the control unit is further adapted to:determine whether a data segment length L=t_(n)−t_(e) between the pointof time t_(n) of an actual data point A_(n) and the point of time t_(e)of a last end point A_(e) exceeds a predetermined threshold length L1and whether the magnitude of the difference between the two differencequotients D_(D)=D₁−D₂ exceeds a predetermined third threshold value T3,and to select the second last data point A_(n-1) as a new end point, ifsaid two conditions are met.
 4. The implantable medical device of claim3, wherein the control unit is adapted to determine the first, and, ifapplicable, the second and the third threshold value as a predeterminedfraction of a peak amplitude value of said time-varying signal.
 5. Theimplantable medical device of claim 4, wherein the control unit isadapted to redetermine the first, and, if applicable, the second and thethird threshold value if a moving average of said peak amplitude valueof said time-varying signal changes.
 6. The implantable medical deviceof claim 1, said implantable medical device having two sensing stagesfor picking-up and sampling of two heart signals, thus generating twodata sequences each representing the time course of a heart signal,wherein the control unit is adapted to select endpoints of both datasequences and to store every selected endpoint of any of the datasequences together with a corresponding data point of the respectiveother data sequence and the segment length between two consecutiveendpoints irrespective the data sequence the endpoints belong to.
 7. Theimplantable medical device of claim 1, wherein the control unit isadapted to transform each magnitude value into a transformed magnitudevalue prior to storing a selected end point, wherein the transformationis nonlinear such that endpoint values with lower magnitudes are encodedwith greater precision, while endpoint values with higher magnitudes areencoded with lesser precision.
 8. The implantable medical device ofclaim 1, wherein the control unit is adapted to downsample an originaldata sequence in order to generate a data sequence comprising less datapoints than the original data sequence, wherein a data point of theoriginal data sequence that has the largest magnitude among a number ofdata points eligible for being dropped is retained and the others aredropped.
 9. The implantable medical device of claim 1, wherein thecontrol unit is adapted to determine the first threshold value as apredetermined fraction of a peak amplitude value of said time-varyingsignal.
 10. The implantable medical device of claim 2, wherein thecontrol unit is adapted to determine the first, and, if applicable, thesecond threshold value as a predetermined fraction of a peak amplitudevalue of said time-varying signal.
 11. A method of deriving, within animplantable medical device, a compressed data representation of a timevarying signal from a sequence of data points that each represent anamplitude A of the time-varying signal at equidistant points of time t,said method comprising the steps of: determining end points of datasegments by a) determination of a first difference quotientD₁=(A_(n)−A_(n-1))/(t_(n)−t_(n-1)) with respect to an actual data pointAn at tn and an immediately preceding, second last data point A_(n-1) att_(n-1), b) determination of a second difference quotientD₂=(A_(n)−A_(e))/(t_(n)−t_(e)) or D₂=(A_(n-1)−A_(e))/(t_(n-1)−t_(e))with respect to an actual data point An at t_(n) or a second last datapoint A_(n-1) and t_(n-1), respectively, and a last end point A_(e) att_(e), said last end point A_(e) being a previously determined end pointor a first data point of said sequence of data points, wherein t_(e)represents the point of time belonging to said end point A_(e), c)selecting the second last data point A_(n-1) as a new end point, if thedifference between the first and the second difference quotientsD_(D)=D₁−D₂ exceeds a predetermined threshold value T, D_(D)>T, anddetermining a data segment length L=t_(n)−t_(e) between the point oftime t_(n) of an actual data point A_(n) and the point of time t_(e) ofa last end point A_(e), and selecting A_(n-1) as a new end point, ifsaid data segment length L exceeds a predetermined maximum length Lmax,and storing all selected end points in association with a segment lengthbetween each stored endpoint and an immediately preceding end point as acompressed data representation of said time varying signal.
 12. Themethod of claim 11, wherein the method further comprises the steps of:determining whether said first and said second difference quotient areof opposite sign and selecting the second last data point A_(n-1) as anew end point, if the magnitude of the difference between the twodifference quotients D_(D)=D₁−D₂ exceeds a predetermined secondthreshold value T2.
 13. The method according to claim 11 or 12, whereinthe method further comprises the steps of: determining whether a datasegment length L=t_(n)−t_(e) between the point of time t_(n) of anactual data point A_(n) and the point of time t_(e) of a last end pointA_(e) exceeds a predetermined threshold length L1 and whether themagnitude of the difference between the two difference quotientsD_(D)=D₁−D₂ exceeds a predetermined third threshold value T3, andselecting the second last data point A_(n-1) as a new end point, if saidtwo conditions are met.
 14. The method according to claim 13, whereinthe method further comprises the step of determining the first, and, ifapplicable, the second and the third threshold value as a predeterminedfraction of a peak amplitude value of said time-varying signal.
 15. Themethod of claim 14, wherein the method further comprises the step ofredetermining the first, and, if applicable, the second and the thirdthreshold value if a moving average of said peak amplitude value of saidtime-varying signal changes.
 16. The method according to claim 11, saidmethod being adapted to process two or more data sequences eachrepresenting the time course of a heart signal, wherein the methodcomprises the steps of selecting endpoints in any of the data sequencesand storing every selected endpoint of anyone of the data sequencestogether with corresponding data points of the other data sequences andthe segment length between two consecutive endpoints irrespective thedata sequence the endpoints belong to.
 17. The method of claim 11,wherein the method comprises a step of value-to-code transformation thatis carried out prior to storing an endpoint or another data pointrepresenting a signal amplitude value, wherein each magnitude value isnonlinearly transformed into a transformed magnitude code such thatendpoint values with lower magnitudes are encoded with greaterprecision, while endpoint values with higher magnitudes are encoded withlesser precision.
 18. The method of claim 11, wherein the methodcomprises the step of downsampling an original data sequence in order togenerate a downsampled data sequence comprising less data points thanthe original data sequence, wherein a data point of the original datasequence that has the largest magnitude among a number of data pointseligible for being dropped is retained and the others are dropped. 19.The method according to claim 11, wherein the method further comprisesthe step of determining the first threshold value as a predeterminedfraction of a peak amplitude value of said time-varying signal.
 20. Themethod according to claim 12, wherein the method further comprises thestep of determining the first, and, if applicable, the second thresholdvalue as a predetermined fraction of a peak amplitude value of saidtime-varying signal.